The invention relates to a hearing aid, preferably to a programmable hearing aid having at least one microphone, at least one signal processor with at least one channel, an output amplifier and an output transducer, at least one of the channels containing a signal processing circuit with at least one percentile estimator for the continuous determination or calculation of at least one percentile value of the input signal from a continuous analysis and evaluation of the frequency and/or amplitude distribution of the input signal, whereby the percentile value(s) serve either directly or indirectly as control signals for controlling the gain and/or the frequency response of the electronic processing circuit, the percentile estimator consisting essentially of a comparator stage with two inputs and two outputs, the first input being directly or indirectly connected to the input of the hearing aid, its two outputs controlling a first control stage the output signals of which control a first integrator, the output of which, directly or indirectly, conveys a control signal to the signal processing circuit and the second input of the comparator stage.
Percentile estimators which may also be used in hearing aids, are known in principle from U.S. Pat. No. 4,204,260.
Clinical tests have shown, that the use of correctly fitted hearing aids, i.e. hearing aids with constant gain, independent of signal-levels, in noisy as well as quiet surroundings are superior to hearing aids with an automatic gain control, with respect to speech comprehension. However, while linear hearing aids require the user to adjust the volume control dependent on the actual listening environment, hearing aids with automatic gain control adapt themselves to the environment and thereby clearly improve the ease-of-use.
Based on the clinical tests mentioned above, the percentile estimators have to work very slowly to achieve an almost constant gain for speech signals. This works very well if one slays in an environment where the level of sound is not varying too much, but the long response times of the system will in some cases not adapt fast enough to changes in environment, resulting in phrases not being heard.
A common problem is the situation where the user of the hearing aid is yelling a message to a distant person. This will increase the percentile estimate and hence reduce the gain in the hearing aid. Since the percentile estimator works slowly, the gain stays reduced for a while, and the hearing aid user will not be able to hear the distant person answering, because the resulting output of the hearing aid will be very low, perhaps even below the user""s hearing threshold level.
On the other hand one could let the percentile estimators work fast, which obviously will make the system adapt faster to changes in environment, but the gain con then not be considered constant for the speech signals. The fast going adjustment of the system will cause xe2x80x9cpumpingxe2x80x9d-effects, which can be very annoying for the user, especially in noisy surroundings, and may result in loss of speech comprehension.
In an automatic gain control system for hearing aids, percentile estimators operating on the present signal in one or more channels may be used for controlling the gain of the electronic signal processors. Such a system is f.i. disclosed in WO 95/15668 (corresponding to U.S. Pat. No. 5,687,241) of applicant.
It is an object of the present invention to create an improved percentile estimator, particularly for use in hearing aids of the kind referred to above, which makes it possible for the hearing aid to adapt fast to changes in the environment, while maintaining a slow response when operating on continuous signals, e.g. speech signals in a steady environment.
This is achieved in a hearing aid as referred to above in accordance with the present invention with a percentile estimator structure having at least a second control stage connected to the first control stage, and at least one additional integrator controlled by the second control stage, the output of which is connected to a further input of the second control stage as well as to a multiplier stage, interconnected between the first control stage and the first integrator.
It is of particular importance that the second control stage supplies a rectified and scaled version of the predefined parameters of the first control stage, generating a positive control signal for the second integrator and a forward reset signal to said second integrator for establishing a predefined minimum value of said integrator whenever the output signal of the first control stage changes.
Other characteristics of the invention and advantageous further embodiments thereof are subject of the remaining claims.